Traditional transducers or loudspeakers (as illustrated in FIGS. 1 and 2 comprise a chassis (13 and 23), usually of metal on which the various active components, a mobile membrane (17 and 27) with its suspension (18 and 28) together with a driver (14 to 16 and 24 to 26) which imparts motion to the membrane. The driver can be of electromagnetic, piezoelectric or electrostatic type. The mobile membrane generates a resulting acoustic wave (11 and 21) in the direction of its displacement (12 and 22) as a function of the electric current which modulates the sound signal to be reproduced. FIG. 1 illustrates in cross section a transducer provided with a membrane of large dimension, more appropriate to the reproduction of medium to low frequencies. FIG. 2 illustrates, in cross section a transducer which is designed more specifically for the reproduction of high frequencies and which is characterized by a smaller membrane, which generally includes a coil, located about its periphery. To this transducer may be added an acoustical horn, enabling an improvement of its output by providing a better acoustic coupling between the membrane (solid) and the ambient environment (air) by a more progressive transition of pressure waves. These transducers of which a great number of variations exist, are chiefly characterized by the direction of the resulting sound wave, which is parallel to the direction of the membrane displacement. Their acoustical radiation is not uniform in all directions with the exception of particular configurations known as pulsating spheres, whose resultant sound radiation is always parallel to the direction of the displacement of the membrane.
Another type of transducer the ESS loudspeaker, invented in the U.S.A. by Dr Oscar Heil (the principle being illustrated in FIG. 3) consists of a folded membrane (37) on which is printed a conductive ribbon; this membrane is positioned in the air gap (36) of a magnetic circuit (35) enabling the magnetic induction to be distributed over the entire membrane. This arrangement, by contraction and expansion (32) of the folds of the membrane, according to the modulating current, results in aspiration and expulsion of the air situated between these folds, and thus in the generation of a resulting acoustic wave (31). in spite of the very high quality of the sound produced, the sound radiation is markedly directional, and furthermore the very weak amplitude of the movements of the folds of this membrane does not enable the reproduction low frequencies.
A third type of transducer, (illustrated in FIG. 4) invented by Sawafugi and Tadashi is a development of an acoustic transducer operating by the deflections of a symmetrical flexible membrane (47). It activates two flexible symmetrical membranes, both attached at one end to a case (43), which are subject to alternating compression and recoil energised by a flat mobile coil (49) located in the air gaps (46) of two magnets (44). The operation of this transducer, being very compact, depends on the symmetrical displacement of the membrane and does not allow any particular directional characteristics.
The auditory spectrum perceptible to man from 20 to 20000 hertz approximately is characterized by the very great variety of wavelength differences involved, ranging from one millimeter to several meters. The reproduction of all these frequencies at acceptable level of power must be achieved by means of two or more loudspeakers, each of which is responsible for a part of the sound spectrum. The result of this necessity is that the acoustic centers of these loudspeakers are several decimeters apart. This degrades the precision and spatial restitution of the stereo signal reproduced and introduces a phenomenon of acoustic interference known as directivity lobes, giving rise to very significant variations of acoustic power emitted depending on the position of the listener in relation to the different transducers. This phenomenon accentuates the undesirable directivity characteristics inherent in traditional loudspeakers.